1. Field of the Invention
This invention relates to a vapor growth apparatus for semiconductor devices, such as compound semiconductor devices. In particular, this invention relates to a vapor growth apparatus for growing thin films from vapor phase by the MOCVD (Metal Organic Chemical Vapor Deposition) method in which a water cooled reaction tube of the vertical type is used.
2. Description of the Prior Arts
The cross-sectional view of a conventional vapor growth apparatus for semiconductor devices is shown in FIG. 6a. This apparatus has a water cooled reaction tube of the vertical type. As shown in FIG. 6b a plurality of semiconductor wafers (substrates) 21 are placed on a susceptor 1 which is in a disk-like form. These wafers 21 are then subjected to the MOCVD method in order to grow thin films on their surfaces.
FIG. 7 is a cross-sectional view showing the structure of the susceptor part contained in the prior art vapor growth apparatus mentioned above. FIGS. 8a, 8b, and 8c are the side view, the plan view, and the cross-sectional view of an integral type heater 102 used in the apparatus.
As shown in FIGS. 7 and 8, integral type heater 102 used in the prior art apparatus is installed in susceptor receiver 11 and connected with current supply terminals 8a and 8b.
Also, as shown in FIG. 6a, the reaction room contained in the apparatus has a shape such as that of a cone, in order to make reaction gas flow most suitable and to obtain the most suitable concentration gradient of the reaction gas. Accordingly, susceptor 1 is close to the lower part of the reaction room, the part which is wider than the upper part. Further, the reaction room is cooled by water so as to prevent the adhesion of products, which are generated by reaction, onto the inner wall. As a result, the thermal radiation from susceptor 1 is relatively large in this apparatus.
In addition, susceptor receiver 11 is placed around susceptor 1 such that receiver 11 holds susceptor 1 from its periphery. This susceptor receiver 11 is integrated into a rotator. Therefore, heat is released due to the thermal conduction from the peripheral part of susceptor 1 to receiver 11.
Moreover, the temperature of the peripheral part of susceptor 1 largely decreases due to the gas flow during a process of vapor growth. As a result, wafer characteristics of products, especially on the peripheral part of susceptor, become inhomogeneous. For example, in producing semiconductor lasers with this apparatus, the wavelength distribution of semiconductor laser oscillation, which is dependent on temperature, becomes scattered.
In order to overcome the problem arising from the temperature decrease, especially in the peripheral part of susceptor 1, heater 102 contained in the prior art apparatus should have a structure to generate more heat from its peripheral part than the inside part. To change the amount of heat generation, the cross-sectional area of carbon heater 102 should be changed. Accordingly, the cross-sectional area of heater 102 should be very small in the peripheral part so as to increase the heat generation, as shown in FIG. 8c. In this case, however, the mechanical strength of this heater decreases.
As explained above, the prior art vapor growth apparatus has the following disadvantages:
(1) the temperature of the peripheral part of the susceptor largely decreases, and thus, the characteristics (including the wavelength distribution of semiconductor laser oscillation) of individual products made of wafers become inhomogeneous; PA1 (2) if the cross-sectional area of the heater is set to be small especially in the peripheral part so as to solve problem (1), the heat generation in this part increases, but the mechanical strength of the whole heater decreases.